1. Field of the Invention
The present invention relates to a manufacturing method of ferrules used in an optical connector for connection and short-circuit of multichannel optical signals in optical transmission. More particularly, the present invention relates to a method of maintaining the structure of a mold for transfer molding and the degree of precision which makes it possible to manufacture a high molecular ferrule, a component part of the optical connector, having a tolerance of less than 1 micron so that it sustain a contact loss of less than 1 dB. Such low contact loss is most important performances measures for optical connectors in connection with the development of a multicore optical connectors where connection and short-circuit of multichannel optical signals are required.
2. Description of the Prior Art
There exists short-circuit in optical cable due to a technical difficulty in manufacture and for proper network construction. Such short-circuit must be connected for the maintenance of optical path. Optical fiber connection must be effected very precisely so as to minimize loss. For such connection, an optical connector which is used where permanent connection by a connector and repeated attachment/detachment are required is now used. A multicore optical connector is less desirable in terms of loss characteristic when compared to a multicore mechanical splice, but more favorable with respect to adaptability and optical cable connection time. It also is preferred for prompt connection of multicore optical cables.
Multicore optical connectors developed thus far make use of basic alignment techniques such as: (1) the method of stacking a single core, (2) the method of connection by forming V-grooves in silicon substrate and the (3) method of inserting an optical fiber into a very small hole molded very precisely. The first method was developed in Japan and MU (Miniature Unit)-type back panel connector made by reducing big monocore ceramic ferrule to a half is typical. This connector does not use an optical fiber array and so it is easy to maintain and repair each line. However, this connector requires precise production technology due to low packaging density and alignment technique using small ferrules and sleeves. Consequently, it gives rise to a relatively great technical difficulty and the price is high.
The second method was developed in the United States. It is widely known as MAC (Multifiber Array Connector).
Though this method is favorable because packaging density is high with a silicon substrate etched so as to form V-grooves and especially multicore can be made possible by stacking, still it requires relatively high production technique necessary to process pitch and height so as to have the degree of precision less than 1 .mu.m, and therefore a bottleneck is caused to price reduction, an important requisite for the connector.
The last method was developed in Japan and MT (Mechanically Transferable) type is typical. According to its use, MTP (or MPO) type is also developed. This connector is most widely used in the world. It incurs a low insertion loss. In the multicore connector realized by this method, ferrule manufacture for alignment is most essential technique and low-pressure transfer molding is used. Since the ferrule uses high molecular molding technique, it is profitable in the aspect of productivity. However, the low loss depends on the accuracy of optical fiber/guide pin guide hole (125 microns/700 microns), pitch (250 microns) and metal guide pin by two kinds of V-grooves, and so accuracy of mold must be less than 0.4 micron and that of metal guide pin less than 1 micron to maintain 1 dB of loss. It is so difficult to realize such an accuracy generally that the mold cost is high and productivity is low accordingly.
Moreover, in the multicore optical connector heretofore in use, a large number of fine holes which have a diameter of 125 microns and tolerance of less than 1 micron are arranged along a line at regular intervals (pitches) with tolerance of less than 1 micron in a high molecular material for the use of optical fiber and, to align two ferrules facing each other for connection, a cylindrical metal stick (i.e., guide pin) precisely fabricated so as to have a tolerance of not exceeding 1 micron, precise pitches formed in ferrules and a metal pin guide hole are used. Consequently, it is necessary to maintain a very high degree of accuracy. However maintaining such a high degree of accuracy increases production cost and lowers productivity. Thus, it is an important cost factor in building a superspeed information communication network now being installed throughout the world.